1. Field of the Invention
The present invention relates generally to chemical sensors that detect changes in analyte concentration or changes in temperature. More particular, the present invention relates to such sensors that incorporate resonant nanoparticles embedded in a matrix that is supported by an optically addressable device.
2. Background of the Invention
Sensors are used to detect the concentration of an analyte and to detect the ambient temperature, among other applications. Chemical sensors based on surface plasmon resonance (SPR) are known. These sensors utilize the enhancement of Raman scattering from a Raman-active analyte that occurs when the analyte is brought near to a plasmon resonant surface. This enhancement is commonly termed surface enhanced Raman scattering (SERS).
The SERS effect is primarily related to the field strength near the surface of the substrate upon illumination, whether the substrate is a roughened metal surface or an aggregate of metallic nanoparticles. The strongest field enhancement is obtainable at the plasmon resonance of the metal substrate or particle. It is for this reason that gold colloid (plasmon resonance=520 nm) is such an efficient SERS enhancer under visible Raman excitation (typically with an argon ion laser at 514 nm).
Chemical sensors that incorporate metal nanoshells embedded in a matrix are described in commonly assigned co-pending patent application Ser. No. 09/616,154, now U.S. Pat No. 6,699,724 filed Jul. 14, 2000, hereby incorporated herein by reference. Metal nanoshells include composite, layered nanoparticles that may include a dielectric core and a metal shell.1,2 Metal nanoshells are described in commonly assigned U.S. Pat. No. 6,344,272 and U.S. patent application Ser. No. 10/013,259, filed Nov. 5, 2001, each hereby incorporated herein by reference. By varying the relative dimensions of the core and shell layers, the optical absorption resonance of metal nanoshells can be controlled and tuned across a broad region of the optical spectrum from the visible to the mid infrared.3 These frequency-agile properties are unique to nanoshells, and promise broad applicability across a range of technological applications.
Enormous increases in the detection sensitivity of molecules via the Surface-Enhanced Raman Effect can be achieved when the molecules of interest are on or near the surface of metal nanoshells and the metal nanoshell resonance is tuned to the wavelength of the excitation laser.4 Enhancements of  greater than 106 with infrared excitation have been observed in highly absorptive solutions, which are equivalent to enhancements of 1012-1014 in thin film geometries where the Raman signal is not reabsorbed. Furthermore, the metal nanoshell resonance can be tuned to the infrared region of the spectrum so that this detection capability can be realized in a region of the spectrum where compact and inexpensive semiconductor laser sources are available. This effect was recently exploited by one of the present inventors in the successful demonstration of an instantaneous immunoassay that can be performed in whole blood with no sample preparation.5 
Nevertheless, despite continuing progress in SPR chemical sensors, there remains a need for a surface plasmon resonant chemical sensor having a controlled optical geometry.
The present invention is based on the discovery that an optical device may be used as a support for a thin film formed by resonant nanoparticles embedded in a matrix. The nanoparticles may be optically coupled to the optical device by virtue of the geometry of the thin film. Further, the nanoparticles are adapted to resonantly enhance the spectral signature of analytes located near the surfaces of the nanoparticles. Thus, via the nanoparticles, the optical device is addressable so as to detect a measurable property of a sample in contact with the sensor.
The optical device may be a reflective device. A reflective device preferably incorporates a reflective surface. For the purposes of the present specification a reflective surface denotes a surface having a ratio of reflectance to transmittance of at least 1.
Alternatively, the optical device may be a waveguide device. A thin film formed by the matrix and the embedded nanoparticles may serve as a cladding layer for the waveguide device.
The measurable property may be the concentration of the analyte in a sample in chemical contact with the matrix, where the chemical contact allows an exchange of analyte between the sample and the matrix.
Alternative, the measurable property may be the temperature of a sample or environment in thermal contact with the matrix, where the thermal contact allows an exchange of heat between the sample and the matrix.
The resonant nanoparticles are adapted to impart ultrahigh sensitivity to the sensor. Further, the sensor can be addressed and read out optically, providing remote sensing capabilities.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings, wherein like numbers refer to like elements.